Vacuum analyzer



.J. G. BACKUS VACUUM ANALYZER 2 Sheets-Sheet 1 Filed July 2, 194.7

f- SPEC TROME' TEE BOX CA THODE lO/V SOURCE POWER w M m H m m SUPPLYCOMPONENTS W/ TH/N MAGNET/C F/ELD COLLECTOR PLATES AMPL /F/E/? IONSOURCE POWER SUPPL Y INVENTOR.

JOHN G BAG/(U5 BY ATTORNEY.

Feb. 28, 1950 J. G. BACKUS 7 9 VACUUM ANALYZER Filed July 2, 1947 2SheetsSheet 2 INvENToR; JOHN G BACKUS BY Win-M ATTORNEY Patented Feb. 281950 NITED STATES P VACUUM ANALYZER Application July 2, 1947, Serial No.758,509

3-Claims. (Cl. 25041.9)

This invention relates to a vacuum analyzer adapted for detecting gasleaks in a highly evacuated vessel and for analyzing the gases presentin said vessel as to their type and their relative concentrations withinsaid vessel, and more particularly to a vacuum analyzer having animproved ion generating source.

In vacuum systems such as the tank portion of the calutron, informationrelative to the qualitative composition of the residual gases present isof great importance in order that ultimate vacuums can be quickly andefficiently obtained. By use of the vacuum analyzer progress of thebake-- out processes and information on other outgassing phenomena canbe simultaneously monitored.

In order to properly analyze the gases present in a vessel undergoingevacuation it is necessary for the analyzer ion source to be capable ofoperating over a Wide range of gas pressures; Dilliculty has beenexperienced with conventional ion sources in this respect, particularlyat low pres sures wherein they are incapable of producing a sufficientsupply of ions and of providing an ion beam having sufiicienthomogeneity of energy to insure proper resolution of the beam. Thisinvention includes an improved ion generating source which when used ina vacuum analyzer is capable of detecting one part of helium to 50 000parts of air at pressures as low as 1 10-- mm. oi mercury contrasted toa sensitivity of one part of helium to 5,000 parts of air at the samepressure when using a conventional type of ion generating source.

A Vacuum analyzer having an ion source slit together with a single slitpositioned on the Ion-"i1 tudinal baflle plate opposite an ion collectorplate will portray for a given nominal ion accelerating voltage,indications of only such ions having comparable mass.

Let us now consider a vacuum analyzer having an ion source slit togetherwith two slits in the longitudinal baflle plate spaced successivelyapart from the ion source slit. Then fora nominal ion acceleratingvoltage, ions of greater mass traverse longer arcuate paths than do ionsof a lesser mass. Consequently in order to admit ions having the lessermass to a collector plate, a slit. in the longitudinal baiile plate mustbe located nearest the ion source slit. Conversely;in order to admitions having the greater mass to a collector plate, a slit in thelongitudinal. baffle plate must be located further from the ionsource-slit than in. the case i or ions having lesser mass.

By incorporating a multiple slit-bafile plate the,

usefulness of the device as a vacuum analyzer is extended to include fora given nominal accelerating Voltage the indication of ions of a widerange of masses.

It is therefore an object of this invention to provide a vacuum analyzerwhereby various gases present in a highly evacuated vessel can bedetected and the relative concentrations of. said gases indicated.

A. further object of this invention is to provide an improved iongenerating source capable of generating from the gas molecules presentin a vessel evacuated to extremely low pressure, a sufficiently abundantsupply of ions to produce satisfactory ion beams for use in the vacuumanalyzer.

A further object of this invention is to provide means for portraying onthe screen of a cathode ray oscilloscope evidences of the relative massand concentrations of ions of gases present in a highly evacuatedvessel, said ions varying widely in relative mass over a range ofapproximately 1 to 50.

A further object of this invention is to provide selective means ofportraying on th screen of a cathode ra oscilloscope evidences of ionshaving relatively low mass, together concurrently with ions ofrelatively higher mass; or, of ions of relatively higher masssingularly;

A further object of the invention is to provide electrical circuitsnecessary to complete the overall operation of the device a a vacuumanalyzer comprising an ion source voltage supply. an ion acceleratorvoltage supply; a pre-amplifier, an amplifier, and a cathode-ray tube.

Other objects and advantages of the invention will be apparent from thefollowing descript on and claims considered together with theaccompanying drawings in which:

Figure 1 is a block diagram of the functional elements comprising thisinvention.

Fig. 2 is a partial schematic representation of certain elements of themass spectrometer as indicated by section line 22 of Fig. 1 showing thedirection of the electromagnetic field, the ion source, and thespectrometer box.

Fig. 3 is a perspective view. partly in section of the massspectrometer, together with an ion generator; mounted on a face plate.

Referring to Figs. 1 and 3, my improved vacuum analyzer generallyindicated at I is suitably mounted on a face plate 2, the. latter beingposiitioned on the wall of a vessel whose vacuum analysis is desiredsuch as a calutron tank. and thereby projecting'the vacuum analyzer iinto the interior of the vessel, and hermeticallysealing the wallthereof. Further, face plate 2 has a gas valve 22 mounted thereon fortest purposes. The vacuum analyzer I comprises essentially thefollowing, all being within the vessel: an electron oscillator type ofion source 3, an ion beam chamher 4, and ion current collector plates I3and I4.

Referring now to Fig. 2, the vacuum analyzer I is positioned in asubstantially uniform magnetic field denoted by the arrows, which isparallel to the longitudinal axis of the ion source 3 and normal to thelarge face of the spectrometer box 4. The ion source 3 consists of ananode cylinder 6 having a pair of electrically connected cathode platesIa and 1b spaced from the open ends of the anode cylinder 6.

Referring again to Figs. 1 and 3, a slit 8 is provided in the wall ofanode cylinder 6 through which ions may be withdrawn and projected intothe box 4 through an accelerating slit 9. There is also provided in thebox 4 a longitudinal baffle A 10 provided with slots II and I2 throughwhich the ion beams may pass and be collected by collector plates I3 andI4 respectively. Moreover, a pair of beam-defining vanes 5a and 5b arepositioned along the 90 radius of the ion beam of greater mass and areso spaced as to trim the beam to an angular divergence of Consideringnow the electrical connections. the cathode plates Ia and lb. and theanode 6 are respectively connected to the negative and positiveterminals of a hi h voltage ion source power supply I8 of the order of2000 volts. Further, the anode 6 and the chamber 4 are respectivelyconnected to the terminals of a sinusoidally varying high volta eaccelerator sup ly I5 which varies from zero to a few thousand volts,the output of the vol age su ply I5 being also applied to the horizontalpla es of an oscillosco e I6. Conside ing the manner of o eration ofrecti-- fier I5 it becomes apparent that after a few cvcles thecondenser becomes charged to a posi ive D. C. potential nominally equalto the secondary voltage of the transformer. The conden er once chargedremains substantially at a D. C. potential due to the R. C. circuithaving a comparatively long time con tant by virtue of t e l rge loadresistance. Consequentlv, on negative eaks of secondary A. C. vol a e. te voltage develo ed in the secondarv winding of the ransformer will beopposed by the equal volta e of the condenser, the resultant output vola e at such instant being zero: further. on positive peaks of secondaryA. C. voltage, the voltage developed in the secondary windin of thetransformer will be additive to the equal voltage of the condenser, theresultant voltage cutout at such instants being double he A. C. peakvalue and o i ive in polarity. Interpolation by this analvsis indicatesthe output voltage to be sinusoidal and ranging from zero to a posit vepotential substantially twice the peak A. C. voltage of the secondary.

Collector plates I3 and I4 are connected as shown (Fig. 1) to a terminalstrip in the preamplifier stage II. Provision is made in said stage I'I,whereby the input thereto can be electrically connected by means of aswitch 2I to collector plate I3 in addition to its connection tocollector plate I4. The output of the preamplifier stage I! is connectedto ground through a resistor I9, the voltage drop across resistor I9being amplified by the amplifier and applied to the vertical deflectionplates of the oscilloscope I6.

In operation, the vacuum analyzer is mounted in the vessel to be testedand the gas in the anode cylinder 6 is of substantially the samecomposition and concentration as the gas in the vessel. When voltagefrom I3 is applied to the electrodes Ia, lb and 6 of the ion source 3,electrons will oscillate between the cathodes 1a and lb through thecavity in the anode cylinder 6 breaking up and ionizing the gases in thecavity, the kind of ion formed being dependent on the gases present,their quantity being dependent on the gas pressure. The ions arewithdrawn through slit 9 into box 4 by the accelerating voltageimpressed between the box 4 and the anode 6. The varying acceleratingvoltage results in the different ion beams sweeping across slots II andI2 and impinging onthe respective collector plates I3 and I4 causingvoltages to be impressed on the vertical plates of the oscilloscope I6proportional to the current density of the particular ion beam.Moreover, the ions traverse circular paths, the radii depending on theimpressed accelerating voltage, the mass and charge of the individualions, and the magnetic field, in accordance with the formula:

H=strength of the magnetic field in gausses; V=the accelerating VOltagein practical volts to which the ions are subjected; r=the radius ofcurvature in inches of the paths along which the ions are projected;

and

=the mass-to-charge ratio of the positive ions projectcd along arcuatepaths and passing transversely through magnetic field H.

The ion egress slots I I and I2 in the baffle surface I0 have beenspaced from the ion source slot 9 so that for a given acceleratingvoltage V and a given magnetic field H, ions of mass two and ten on thescale of O+=16 will pass through their respective ion egress slots,whereby the ration of their radii of curvature becomes This arrangementprecludes ambiguity in differentiating between the Hz ion and the HzO+ion which otherwise would occur were the slots II and I2 positioned inaccordance with ion masses two and eighteen. More particularly, slots IIand I2 are so spaced from slot 9 that when an ion beam passes throughslot II no ion beam passes through slot I2, thus avoiding overlappingpeaks on the oscilloscope I6. Inasmuch as the oscilloscope sweep iscontrolled by the accelerating voltage, the oscilloscope trace is anindication of the various ion densities, the abscissa identifying theindividual ion beam and the ordinate the density thereof, the formerbeing an indication of the gases present and the latter theconcentrations thereof.

Thus, after the system has been practically degassed, the presence of ahigh HO+ peak is an indication of water vapor pointing to a water leakwhile a high N+ peak would indicate an air leak. By directing a streamof hydrogen at a suspected surface region of the vessel a rise in the Hpeak would indicate a leak in this region.

Although this invention has been described with reference to aparticular embodiment thereof, it is not limited to this embodiment norotherwise except bythe terms of the following claims.

What is claimed is: v i Y 1. Inamass spectrometer anion generatingsource comprising a pair of spaced cathodes having a gaseous regiontherebetween, an apertured tubular anode axially aligned with, spacedfrom, and intermediate said cathodes, said anode being positioned withinthe magnetic field of said spectrometer and axially aligned therewith.

2. A vacuum analyzer comprising a mass spectrometer having an ionsource, a spectrometer box, and a receiver, said ion source comprising atubular apertured anode disposed in and parallel t0 the magnetic fieldof said mass spectrometer and a pair of cathodes disposed adjacent theends of said anode, said spectrometer box being positioned adjacent saidion source and having an ion access slot formed therein, and beingfurther provided with at least two ion egress slots, said receiver beingdisposed exterior to said spectrometer box and adjacent said ion egressslots, and electrical elements cooperatively associated with said massspectrometer providing a visual indication of the kind and amount ofions formed at said ion source and received at said receiver.

3. A vacuum analyzer comprising a 180 degree mass spectrometer having anion source, an ion \2 receiver, and an ion beam therebetwee'n, meanssupplying a sweep voltage to said mass spectrometer whereby thetrajectory of said ion beam is varied, and an ion baflie plateinterposed between said receiver and said ion beam situated on thediameter of said ion beam path and having two ion egress aperturesformed therein, the ratio of distances from said ion source to saidapertures being substantially JOHN G. BACKUS.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,146,025 Penning Feb. '7, 19392,331,189 Hipple Oct. 5, 1943 2,341,551 Hoover Feb. 15, 1944 2,355,658Lawlor Aug. 15, 1944 2,370,673 Langmuir Mar. 6, 1945

